Field of the Invention
The invention is related to a method of surface modification of alumina.
Description of the Related Art
As a catalyst carrier, alumina has the following disadvantages. First, the mechanical strength of alumina, especially its abrasion resistance, is rather low. In a continuously stirred slurry bed reactor or circulating fluidized bed bioreactor, the carrier tends to break thereby shortening the service life thereof and increasing the difficulty to separate products from catalyst powders. Second, the acid resistance of alumina is poor. Under acid environment, the surface of alumina is easily dissolved thereby weakening the connection between the active metal and the carrier. Therefore, the service life of catalysts and the product quality are low. Third, the carried active metal has strong interaction with the alumina carrier. When calcining the catalyst, an aluminate compound having a spinel structure is formed, which is very difficult to decompose. Thus, it is necessary to modify the surface of alumina carrier to improve its chemical and physical properties.
Silicon nitride has good properties, such as abrasion resistance, chemical inertness, acid resistance, and thermal conductivity. However, ordinary materials made from silicon nitride have relatively small specific surface area (below 1 m2/g), which is usually insufficient for carrying active metals. In addition, the synthesis temperature of silicon nitride generally exceeds 1200° C., which is far beyond the crystal transition temperature of most alumina carriers, and the resulting primary product contains complex impurities. Thus, conventional methods cannot modify the surface of an alumina carrier by use of silicon nitride.